Producing sand with well fluids from an unconsolidated subterranean oil or gas producing zone has long been a problem in the petroleum industry, causing erosion of production equipment and plugging of the well. Such conditions often result in reduced production levels or loss of well production entirely.
An effective means of combating the problem is the gravel pack, which involves placing a tubular liner in the well bore and packing gravel in the annulus between the liner and the well bore. The liner has slots or other apertures in its walls which are smaller in size than the gravel particles, thereby permitting formation fluids to flow through the slots while preventing entry of the particles. The small apertures may be provided by a screen encircling the outer circumference of the liner tube, in which case the openings in the tube may be larger than the gravel particles. As a result of improved gravel pack technology, gravel packs have become quite effective in excluding sand from oil and gas production. In addition to this function, the gravel also assists in supporting the walls of uncased wells and preventing caving of loose material against the liner. Despite the effectiveness of gravel packs once they are properly placed and operating, it is often difficult to install a uniform gravel pack due to the problem of premature bridging.
Gravel is commonly mixed with a fluid, such as a liquid or foam, to form a slurry which is introduced through a work string and a crossover tool into the annulus between the well bore and the liner. The slurry flows down the annulus to the bottom of the well bore or to a sump packer in the well bore. Some of the fluid of the slurry flows through the apertures in the liner into the open bottom end of a wash pipe situated within the liner and returns to the surface through the crossover tool and the annulus between the work string and the well casing. The balance of the slurry fluid flows into the subterranean zone through perforations in the well bore. Gravel is thus deposited in the annulus and against the subterranean zone.
Premature bridging can occur when the upstream perforations, that is, the perforations first encountered by the gravel slurry, fill with gravel from the gravel slurry and form deposits which impede the flow of the slurry down the annulus. When this occurs, the slurry follows the path of less resistance, which in this case is through the screen. This in turn accelerates the bridging action, as flow of the slurry fluid through the screen deposits gravel on the screen. While bridging in this manner can occur in a vertical well, the problem is more severe in a deviated well, where gravel transport through the horizontal well bore annulus is more difficult because it is not assisted by gravity.
A number of suggestions have been made over the years as to how to prevent bridging. One suggestion involves the use of a large diameter wash pipe, which reduces the size of the annulus, or the annulus clearance, between the wash pipe and the liner, thus increasing the resistance to flow through the annulus. This causes the carrier fluid to preferentially flow down the annulus between the well bore and the liner rather than pass through the liner openings. This method is not very effective, however, in limiting flow behind the screen and in preventing bridging at the top of the screen.
Another suggestion involves plugging the liner perforations with cement or other material prior to introducing the liner into the well bore, and then later removing the material by an acid treatment or some other separate removal operation. Complicated liner designs involving special liner flanges or valves at every liner joint have also been proposed. In addition, it has been suggested to provide material such as a semi-solid gel between the wash pipe and the liner in order to temporarily plug the liner openings until the top of the gravel pack reaches the openings. At that point the semi-solid gel offers less resistance to flow than the gravel pack itself, resulting in the liquid breaking through the gel and flowing down to the open end of the wash pipe. Still another method involves the use of a liner to seal the casing perforations and then carrying out the packing operation through use of new perforations. These various methods, however, either slow the gravel packing operation or make it less efficient. In the case of the method utilizing semi-solid gel to temporarily plug the liner openings, the gel is not designed to withstand the higher pressures sometimes encountered at locations other than at the perforations adjacent the top of the gravel pack, and so cannot be relied upon under all operating conditions.
It would be very beneficial to be able to reliably and consistently install gravel packs corresponding to gravel pack design parameters. It is therefore an object of the invention to prevent premature bridging in a gravel packing operation in a manner which does not impair or slow the gravel placement process.